Currently, rc.inidiskless assumes that local system configuration
changes are kept in some mountable file system. For example,
nanobsd uses dedicated partition mounted as /cfg for this.
However, small embedded devices like MIPS routers may have no enough flash
space to keep full-blown file system but have only one or couple
small flash blocks to keep persistent local configuration overrides.
This change extends rc.initdiskless and introduces ability to run auxiliary
command /conf/T/M/extract that is supposed to extract configuration overrides
from such local storage.
For example, the command /conf/default/etc/extract may contain something like:
cd "$1" && bsdcpio --quiet -idu < /dev/map/cfg
bsdcpio command extracts compressed archive from the storage to /etc
assuming the storage is exposed by the kernel as /dev/map/cfg to userland.
PR: 204215
MFC after: 1 month
b4d24263c2